It is desirable to fabricate fully dense silicon nitride because high temperature strength and other physical characteristics will be maximized. Full density usually means 100% of theoretical, but herein shall include densities in excess of 99% of theoretical. Most commercial methods for fabricating fully dense Si.sub.3 N.sub.4 based materials employ the process of hot-pressing which involves simultaneous application of pressure and elevated temperature to achieve high density and consequently superior physical properties. Unfortunately, this process is costly and significantly limits the ease with which certain configurations of such materials can be formed, particularly cutting tool inserts. In hot-pressing, a large block or tile of the material is initially hot-pressed into a simple rectangular or disc-shaped article from which a tool insert is individually cut by a slow, tedious and costly method of diamond sawing. Holes, chip breakers, positive rake angles, etc., must be ground in later at great expense. Finish grinding of the hot-pressed insert to its final shape is also more tedious and costly since all corner radii must be ground into the insert starting with a totally sharp corner.
Thus, it would be important if these materials could be fabricated by a method which did not employ hot-pressing and was considerably less expensive. Some effort has been made in this direction by attempting to sinter silicon nitride powder into dense shapes by heating in the absence of applied pressure. This is pointed out particularly in U.S. Pat. No. 3,992,497. However, in each instance where sintering has been attempted to densify silicon nitride material, the ultimate density has been less than satisfactory and certainly below a fully dense object. For example, in U.S. Pat. No. 3,992,497, the highest density achieved ranged from 69-95% of full density. In U.S. Pat. No. 3,953,221, the density never exceeded 97% of theoretical. This is far short of the fully dense compact required herein. Others have experimented with a variety of mechanisms to assist such sintering in the hope that a higher density and strength could be achieved equivalent to hot-pressing. Examples of such mechanisms include: (a) a fibrous structure sought in U.S. Pat. Nos. 3,969,125 and 4,101,616, each of which did not achieve densities in excess of 96-97%, (b) an amorphous powder in U.S. Pat. No. 4,073,845 but the highest density achieved was 96% with the average density being considerably lower, (c) a coating of silica and boric oxide in U.S. Pat. No. 3,811,928 to improve the sintered object, the best density being 95%, (d) spinels to enhance the sintered object, such as in U.S. Pat. Nos. 3,950,464; 4,004,937; 4,026,351, each of which achieved densities in the range of 85-95%, and (e) compounds of silicon, aluminum, oxygen and nitrogen (SiAlON) such as illustrated in U.S. Pat. Nos. 4,127,416, 3,989,782 and 3,991,148, each of which achieved densities in the range of 96-97%. These attempts by the prior art to achieve pressureless sintering of a fully dense silicon nitride compact did not succeed.